The present invention relates to a transmission method according to the preamble of claim 1, to a transmission apparatus according to the preamble of claim 5, a receiving method according to claim 9, a receiving apparatus according to claim 13 and a transmission system according to claim 17.
In a conventional OFDM-system signals or information contained in signals are modulated onto subcarriers in the frequency domain. The spacing between the subcarriers is equal and the subcarriers are arranged orthogonally in the frequency domain. The respectively applied modulation scheme varies for example the magnitude and phase of the described subcarriers. A conventional transmission apparatus for transmitting OFDM-signals therefore comprises as basic elements modulation means for modulating said signal onto a plurality of subcarriers using a OFDM-modulation method, transformation means for transforming said modulated signals into the time domain, and transmission means for transmitting said signals. In a conventional OFDM-system, a transmission means for OFDM-signals extends a time domain signal after a transformation into the time domain (e. g. by an inverse discrete Fourier transformation) by some guard samples to overcome multipath effects during the transmission. Usually the extension of the time domain signal is done by a cyclic extension, wherein a part of the wave form is repeated. A corresponding OFDM-signal receiving apparatus can perform correlation utilizing the two identical wave form parts to obtain information on the timing of the OFDM-time bursts for further processing. Usually this timing information is used to optimally place the discrete Fourier transformation window in the receiving apparatus to be able to transform the modulated subcarriers into the frequency domain and to demodulate them thereafter.
To provide an efficient transmission system, the guard time or cyclic extension has to be as short as possible, namely slightly larger than the longest expected transmission path duration, which can result in poor cyclic extension based correlation properties in a receiving apparatus if the cyclic extension is very short (e. g. only a few samples). In this case, in known OFDM-systems synchronization bursts are used, which contain only synchronization information. This reduces the transmission efficiency, since a special synchronization burst designed in the time domain does not contain information (in the frequency/subcarrier domain) to be transmitted.
The object of the present invention is therefore to provide a transmission method according to the preamble of claim 1, a transmission apparatus according to the preamble of claim 5, a receiving method according to claim 9, and a receiving apparatus according to claim 13, which provide optimized correlation possibilities.
This object is achieved by a transmission method according to claim 1, a transmission apparatus according to claim 5, a receiving method according to claim 9, and a receiving apparatus according to claim 13. Also, this object is achieved by a transmission system according to claim 17.
The transmission method for transmitting OFDM-signals according to the present invention comprises the steps of modulating said signals onto a plurality of subcarriers using a OFDM-modulation method, transforming said modulated signals into the time domain, and transmitting said signals, characterized in that in said modulating step every M-th subcarrier is modulated with a signal, wherein M is an integer and Mxe2x89xa72.
The transmission apparatus for transmitting OFDM-signals according to the present invention comprises modulation means for modulating said signals onto a plurality of subcarriers using a OFDM-modulation method, transformation means for transforming said modulated signal into the time domain, and transmission means for transmitting said signals, characterized in that in said modulation means every M-th subcarrier is modulated, wherein M is an integer and Mxe2x89xa72.
The receiving method according to the present invention is adapted for receiving OFDM-signals comprising M identical or respectively mirrored wave forms within one OFDM-timeburst, wherein M is an integer and Mxe2x89xa72, and comprises the steps of receiving said OFDM-signals, correlating said wave forms to obtain time synchronization, transforming said signals into the frequency domain, and demodulating said signals.
The receiving apparatus according to the present invention is adapted for receiving OFDM-signal comprising M identical or respectively mirrored wave forms within one OFDM-timeburst, wherein M is an integer and Mxe2x89xa72, and comprises receiving means for receiving said OFDM-signals, correlation means for correlating said wave forms to obtain time synchronization, transformation means for transforming said signals into the frequency domain, and demodulation means for demodulating said signals.
Advantageous features of the present invention are defined in the respective subclaims.
The modulation of every M-th subcarrier according to the present invention, after the succeeding transformation of the signals into the time domain, e. g. by an inverse discrete Fourier transformation, results in a signal containing M identical or respectively mirrored wave forms, whereby the total duration of the OFDM-timeburst is still 1/f0 (f0 is the subcarrier spacing). With M identical wave forms within one OFDM-timeburst, the corresponding receiving apparatus can perform an optimized correlation in the time domain, e. g. to obtain time and frequency information and synchronization, respectively. Further on, information to be transmitted can be modulated onto every M-th subcarrier and the transmission of a special time-domain synchronization time burst usually not containing useful information in the frequency-subcarrier domain is not necessary.
The present invention can be applied to every transmission system based on a multicarrier OFDM-modulation method, e. g. wireless and wired transmission systems. Possible and advantageous applications of the present invention in a wireless transmission system are for example the RACH (Random Access Channel), the BCCH (Broadcast Control Channel), and the IACH (Initial Acquisition Channel). Generally, the present invention is especially effective in scenarios where conventional algorithms to improve correlation based time synchronization, e. g. averaging over multiple time bursts is not possible. The present invention can be applied to any OFDM-system, particularly, when a robust time synchronization for further signal processing, e. g. discrete Fourier transformation, is required.
Advantageously, in said modulation means the not modulated subcarriers are set to zero. Further advantageously, only subcarriers with even indices are modulated. If only subcarriers with even indices are modulated (e. g. M=2), a full (complex) time domain signal consisting of two equal wave forms is obtained after the transformation into the time domain (e. g. by an inverse discrete Fourier transformation). If, on the other hand, only subcarriers with odd indices are modulated (e. g. M=2), a full (complex) time domain signal after the transformation into the time domain is obtained, which contains two respectively mirrored wave forms. In this case, the two wave forms are mirrored so that the correlation result is negative and an additional absolute value unit (or inverter) is necessary in the receiving apparatus to achieve a positive correlation result and a correct frequency offset.
Advantageously, said modulation means comprises means for generating integer values from 0 to Lxe2x88x921, wherein L is the number of available subcarriers, whereby said modulation means modulates every M-th signal onto said subcarriers on the basis of said integer values.
Advantageously, in the correlation means of the receiving apparatus according to the present invention, the identical or respectively mirrored wave forms are correlated on the basis of a delay value L1=S/M and averaged over L2xe2x89xa6S/M samples, whereby S is the total number of samples in one OFDM-timeburst.
It is further advantageous in the receiving apparatus according to the present invention to provide a peak detection means after said correlation means for providing time synchronization for the transformation of said signals into the frequency domain. It is further advantageous to provide a frequency offset detection means after said correlation means for providing frequency synchronization for the transformation of the signals into the frequency domain.
The transmission system for transmitting OFDM-signals according to the present invention comprises a transmission apparatus according to the present invention and a receiving apparatus according to the present invention. This transmission system can be based on a wireless or wired transmission of signals.